The present invention relates to a method for improving the accuracy of a rapid fire weapon by determining a change in muzzle velocity as the gun barrel is eroded during use.
One value which is required in an equation of a trajectory is initial velocity, which is the velocity with which the projectile is supposed to leave the muzzle of the gun. The term muzzle velocity is synonymous with initial velocity. The initial or muzzle velocity realized from a conventional gun is controlled principally by six factors:
1. the ratio of propellant charge weight to projectile weight; PA1 2. the ratio of total gun volume to chamber volume, i.e., expansion ratio; PA1 3. the thermochemical properties of the propellant; PA1 4. the physical dimensions of the propellant; PA1 5. bore friction; and PA1 6. ignition phenomena.
In the case of a military weapon, such as a 5 inch gun, factors 1, 3, and 4 are held constant, but the expansion ratio varies as the gun becomes worn and the seating position of the projectile changes. This causes the chamber volume to increase, while total gun volume remains constant. Bore friction and ignition system variation introduce a significant amount of random variation into the firing process and accounts for the round-to-round variation within a given ammunition lot.
By way of example, a 5 inch gun presently in use by the military departments is a high performance rapid fire weapon capable of sustained firing at rates up to 40 rounds per minute. A method of muzzle velocity calibration was developed around a bore erosion gauge which is used in association with a velocity loss table. The bore erosion gauge depends upon finding a 5.025 inch diameter in the erosion slope nearest the breechface and the correlation between this location and mean muzzle velocity performance. The original velocity loss table was based upon a series of barrel life tests conducted during the development of the 5 inch gun system. In these tests a standard firing schedule was used consisting of firing 100 rounds at 40 rounds per minute with 20 second pauses after each 20 round burst. This was followed by complete cooling of the barrel. This standard barrel life schedule represented a worst case erosion condition.
Because of the low rate of peace time fleet usage, the inadequacies of the standard calibration technique did not readily become evident. Typically, the gun barrels in the fleet were subjected to only the slow spotted direct fire of shore bombardment training exercises or the very few rounds fired at maximum rate against air targets. There were no sustained rapid firing with attendant hot gun conditions were residual bore temperatures exceed 250.degree. F., and, consequently fleet training exercises represented a best case erosion situation. It later developed that Navy operational units were consistently overshooting their targets during training exercises and the source of error was traced to the inability to establish an accurate muzzle velocity calibration using a bore erosion gauge.
Prior to and during the Viet Nam War, the standard Naval gun propellant in use was a medium flame temperature (2,450-2,650.degree. K.) composition known as PYRO. This propellant, when used in a 5 inch gun led to a very short barrel life (average of 1500 rounds). The demamds for Naval gun fire support in the Viet Nam War quickly demonstrated that such a short barrel life was unacceptable. Accordingly, a new low flame temperature (2,180.degree. K.) propellant known as NACO was developed which extended barrel life by a factor of 4 to 6. This new propellant, however, produced greatly different erosion characteristics and the utility of the erosion was further degraded.